Previously, a wide variety of rotational position detectors have been proposed. These detectors may be roughly divided into two groups. The first group includes optical or magnetic pulse encoders, which output detected information on rotational position in the form of pulse signals. The second group includes optical or magnetic resolvers, which convert the angular information into analog signals using a sine or cosine function.
A photoelectric conversion-type of rotational position detector has previously been proposed, and is illustrated in its essentials in FIGS. 4(a)-(e). FIG. 4(a) illustrates the primary components of the position detector. A rotating shaft 1 of a rotating body (not shown) extends concentrically through a light emitting member 2, such as a diode, and is joined to a light shielding disk 3. Light shielding disk 3 includes a light transmission window 31, which permits light from light emitting member 2 to be transmitted to a light receiving member 4 having a light receiving portion 41.
Referring to FIG. 4(a), light receiving disk 4 includes, for example, a semicircular light receiving portion 41. Light receiving disk 4 is fixedly disposed, and is spaced a predetermined distance from light shielding disk 3 to avoid contact between light emitting disk 4 and light shielding disk 3. Light transmission window 31 permits transmission of light over an area of light receiving disk 4 defined by the rotation through 180 degrees from a predetermined reference axis passing through the central axis of rotating shaft 1.
FIGS. 4(b)-(d) illustrate various transitions that occur as light shielding disk 3 rotates with respect to light receiving disk 4. In FIG. 4(b), o is a reference axis on light receiving disk 4, and .beta. is a reference axis on light shielding disk 3. The angle .theta. is the angle between the axes .alpha. and .beta. and therefore varies as light shielding disk 3 rotates, as shown in FIGS. 4(b)-(d). If counterclockwise rotation (denoted CCW in the Figures) is designated as the forward direction of light shielding disk 3, the exposed areas 41A, 41B and 41C of the light receiving portion 41 varies with the rotation of the light shielding disk 3. This area is represented by S, and increases as light shielding member 3 rotates CCW from .theta.=0, where S=0, to .theta.=.pi., where S is maximized. As .theta. increases from .pi. to 2.pi., S decreases proportionally until S=0. This variation is shown graphically in FIG. 4(e).
If light receiving portion 41 is formed from a photoelectric conversion material, such as amorphous silicon, for generating an electric current in proportion to the area that is exposed to light, the output short circuit current I.sub.sc is generated in quantities proportional to the area S of exposed light receiving portion. This is also illustrated in FIG. 4(e). Rotational positions, measured as angles, of the rotating object can therefore be determined by measurement of the output short circuit current I.sub.sc.
One embodiment of such a resolver is shown in FIG. 5(a). Two light receiving portions 42 and 43 are shaped such that the size of the area S viewed through a light transmitting window (not shown) of a light shielding disk is a sine function of the angle. Light receiving potions 42 and 43 are each disposed to be spatially 90 degrees out of phase from the other. Two sinusoidal outputs I.sub.sc1 and I.sub.sc2 are thereby obtained, having a phase difference of ninety degrees, as shown in FIG. 5(b).
FIG. 6 shows another embodiment, in which the light receiving portions are configured to obtain four sinusoidal output signals each differing in phase by ninety degrees. Light receiving portions 44E, 44F, 44G and 44H are connected respectively to I.sub.sc output electrodes 45E, 45F, 45G and 45H by conductors 46E, 46F, 46G and 46H, respectively, made from the same photoelectric converting material as each light receiving portion. A metal mask 47 having windows 47E, 47F, 47G and 47H corresponding to the light receiving portions is placed over the conductors to prevent the generation of electric power in the conductors by exposure to light.
Position indicators of the types described have several inherent problems. First, they require predetermined spacing between light receiving portions such that one light receiving portion is not connected to another. All light receiving portions are therefore of different shapes, and differ from each in amplitude as well as in phase. Second, the spacings as to outer light receiving portions must be made wide enough to accommodate conductors (such as 46G and 46H of the embodiment shown in FIG. 6) leading from inner light receiving portions (such as 44G and 44H). Third, when the number of light receiving portions is increased to obtain a greater number of phase differentiated output signals, the constraints just described become even more limiting.
A fourth problem associated with the previously described embodiments lies in the positional adjustment of the metal mask (such as 47 in FIG. 6). Such adjustments must be made in both the radial and circumferential directions to assure proper alignment.
Finally, the conductors (such as 46E, 46F, 46G and 46H) typically are of differing lengths. As a result the impedances introduced by such conductors differ from each other.
These problems result in difficulty in the miniaturization of photoelectric conversion-type position indicators and improvements in the efficacy of such devices.